Material processing is used to adjust the performance of electronic elements by removing or otherwise affecting a portion of the material of the electronic element to change the electrical characteristics thereof. It is known to change the electrical properties of passive and some active electronic elements by removing material therefrom. Methods of removing material include applying laser energy for vaporizing a portion of the material, applying laser energy for ablative removal of the material and applying laser energy to affect a photochemical reaction for removing and/or otherwise altering an electrical performance characteristic of the material. It is well known that the relative effect of these three processes depends on the energy density and wavelength of the laser, and the properties of the material illuminated by the laser.
Lasers are used to perform all of these material-processing techniques. Laser material processing is routinely performed using a position and power controlled laser beam that is directed to scan over a desired region of the material for processing. These techniques are used to process individual passive electronic elements such as resistors, capacitors and inductors, as well as to process electrical elements in microchips, e.g. for memory chip repair and/or for trimming electrical elements formed onto silicon or other crystalline substrates.
In particular, a laser beam is directed over a region of the electrical element to remove or trim material from the element. The trimming may affect the electrical performance of the element by reducing the volume of electrical material in the element or by altering a path of electron flow through the material, e.g. by creating a longer resistive path or even by creating an open circuit by completely removing a conductive path between two elements.
It is well known in the manufacturing of precision electrical resistors to laser trim each resistor to adjust its resistive value to fall within a desired range. It is also known to measure the resistive value during the laser trimming process and to continue to trim the resistor until the resistive value is acceptable.
FIGS. 1A and 1B depict a conventional precision chip resistor manufacturing process wherein a resistor panel 100 comprises a ceramic substrate 110 having a plurality of precision resistor elements 120 formed thereon by a printing, screening, growing or any other resistor forming process. The resistor panel 100 may also include conductive portions formed thereon including conductive leads (not shown) attached to each resistor 120 for providing an electrical connection to the resistor element. Conductive measurement contacts 130 may also be provided for probing the resistor element 120 to measure its resistance value.
FIG. 1B depicts an enlarged resistor element 120 having conductive measurement contacts 130 attached thereto for providing a convenient resistance measuring position for placing measurement probes 140 in electrical contact with the resistor element 120 for measuring its resistance value. A measurement circuit 150 is connected to the probes 140 and applies a measurement voltage across the resistor element 120. It is customary that the probes 140 measure the resistance value during the trimming process.
According to conventional precision chip resistor manufacturing methods, it is desired that each of the resistor elements 120 formed on the ceramic substrate 110 have substantially the same resistance value within a narrow tolerance range. However, each resistor element 120, as applied to or formed onto the ceramic substrate 110 may have a resistance value that falls outside of the desired range due to uncontrollable aspects of forming the resistive elements onto the substrate. Accordingly it is known to trim each resistor using a laser-trimming device to adjust the resistance value, if needed, such that all of the resistor elements have a resistance value that is within the desired range. To trim each resistor element 120, the probes 140 are electrically contacted onto the measurement passive elements 130 and the initial resistance value is measured. A laser beam is then directed onto the resistor element being probed to make a trim cut which cuts through the resistor material to the ceramic substrate. The cut resistor has an increased electrical resistance as compared to the uncut resistor, as the cut reduces the cross-sectional area.
Various resistor trimming cuts are known, including a double plunge cut shown in FIG. 2A and an L cut shown in FIG. 2B. In each of these cuts it is desirable to adjust the resistance value without locally restricting the current flow so much that the region of the resistor around the trim cut is excessively heated by ohmic losses during normal operation of the finished circuit. Accordingly, the cuts are made in a central region of the resistors without cutting more than about half way across the resistor width, W.
Once the trimming is complete, the panel 100 is diced up into sections with one resistive element 120 on each section and the resistive elements are incorporated into electrical circuits usually as surface mounted elements. Accordingly it is known in the prior art to manufacture individual resistors, which are trimmed before being incorporated into an electrical circuit or installed onto a PCB panel. These resistors may be surface mounted onto printed circuit boards (PCB's) such as computer mother boards, cell phone controllers, and the like, or the resistors may be used in transducers, testers, controllers or any other type of electrical device where a pre-measured pre-calibrated resistor may be required.
Similarly, many of the techniques described above are used during the manufacture of precision chip capacitors. In FIG. 2C, a typical capacitor cut is shown wherein a corner of the capacitor is isolated from the remaining element to adjust the area of one of the conductive layers, thereby reducing the capacitance of the element.
One example of a precision chip element laser trimmer currently available is the GSI LUMONICS W770 Chip Element Trim System, manufactured and distributed by the assignee of the present invention.
In another example of the use of laser trimming, it is known to trim individual elements such as resistors, resistor networks, capacitors, inductors and integrated passive elements that are incorporated into a hybrid integrated circuit formed, for example, on a ceramic substrate. As described above, individual elements of the hybrid circuit may be probed to measure an electrical characteristic of the element and laser trimmed to adjust the electrical characteristic as required. The finished tested and trimmed circuits may then be sold for incorporation into another device, e.g. mounted onto a PCB, or the hybrid circuit module itself may comprise a special purpose device such as a sensor.
Examples of hybrid circuit trimming systems currently available for laser trimming resistors include the GSI LUMONICS W670 Thick Film Laser Trim System and W678 Thin Film Laser Trim Systems each manufactured and distributed by the assignee of the present invention.
In other examples of laser trimming, it is known to trim individual elements such as chip resistors, resistor networks and integrated passive elements that are incorporated into an integrated circuit formed, for example, on a silicon substrate or other semiconductor. As described above, individual elements of the circuit may be probed to measure an electrical characteristic of the element and laser trimmed to adjust the electrical characteristic as required. The finished tested and trimmed semiconductor circuits may then be sold for incorporation into another device, e.g. mounted onto a PCB, or the circuit chip itself may comprise a special purpose device such as a transducer.
It is also known to trim elements after an element has been surface mounted onto a PCB or otherwise incorporated into an electrical circuit. In this case, a completed and functional device may be probed to measure an electrical response to an electrical input stimuli and one of the circuit elements of the electrical circuit may be laser trimmed while the circuit is active, to change the circuit performance according to a desired circuit response.
Other electronic elements such as capacitors and inductors have also been trimmed using a laser to remove or otherwise affect a material that can change the capacitance or inductance, respectively, of the element. It is known to trim individual passive elements such as resistors, capacitors and inductors to achieve a desired resistance, capacitance or inductance. It is also known to trim passive elements in a passive network of elements for example to adjust the resistance of a pair of resistors connected in parallel or in series by trimming just one of the resistor elements.
As shown in FIG. 3, historically, pre-manufactured trimmed electronic elements and even trimmed integrated circuits mounted on silicon chips have been installed onto the surface of a prefabricated PCB panel. A section of a finished PCB 160 of the prior art is shown in FIG. 3. Passive electronic elements 170 and 180, such as pre-trimmed resistors, capacitors, inductors or chips, are attached to a PCB panel and interconnected with other circuit elements via conductive layers 190 which provide predetermined conductive paths between surface mounted elements. Each conductive layer is separated by a dielectric layer 200 to electrically isolate the conductive layers.
Passive electronic elements may electrically contact just the top conductive layer 190, as shown for the element 170 (called surface mount components), or a component may be mounted using one or more holes drilled completely through the printed circuit board (called through hole components), possibly connected to several conductive layers 190 depending on the individual circuit design. In this latter case, the element 180 includes conductive leads 210 that are inserted into through-holes 220 drilled or otherwise formed in the PCB blank prior to mounting the element 180. In any case, the elements are attached and electrically contacted to conductive layer(s) by soldering.
Thus according to the prior art, a finished PCB includes one or more conductive layers and one or more dielectric layers. The conductive layers are generally etched or otherwise formed into a pattern of conductive paths that interconnect to form electrical circuits. Conducting via holes are pre-formed through the one or more conductive and dielectric layers to allow a surface mounted element to form an electrical contact with one or more conductive layers. Thereafter, passive or active electronic elements of the circuits such as resistors, capacitors, inductors, chips, transistors, amplifiers, diodes, and the like, are surface mounted onto the PCB blank and soldered to contact one or more of the desired conductive layers. In many cases, the electrical elements are pre-trimmed to provide a specific electrical performance. However, in some cases, a surface mounted element may be trimmed after installation onto the PCB.
According to newly developed procedures for fabricating finished PCB's, some passive elements are being formed directly onto the conductive and or dielectric layers of a PCB panel such as is shown in FIG. 4. FIG. 4 depicts one example of a partially fabricated PCB panel 240 having a dielectric layer 250 and a plurality of conductive paths 260 formed onto the dielectric layer 250 by conventional means.
As shown, the substantially same circuit pattern 270 is repeated six times over the panel such that six separate circuits may be formed on the single panel 240. After completion, including the mounting of any surface mounted elements, the panel 240 may be diced up into six separate PCB's. Alternately, the panel 240 may comprise one large circuit board.
In the present example, a plurality of passive resistors 280 are formed directly onto the panel 240 by printing, painting or otherwise applying a resistive paste or film between selected conductive paths 260 of the circuit 270. Each resistor is in direct contact with the dielectric layer 250 and interconnects at least two isolated conductive paths 260. As an alternative process, a sheet of resistive or insulating material backed with a conductive layer is laminated as a continuous layer on the circuit board, and then the patterns are etched in the conductive/resistive or conductive/insulating structure to form resistors or capacitors. Both the printing and etching techniques for forming embedded passive elements are discussed in greater detail following a discussion of the present limitations of embedding passive elements in printed circuit boards.
All of the passive resistors 280 may be applied simultaneously such as through screen or template or they may be applied in several steps. After application, the resistive paste or liquid is cured by baking or another curing process to provide a finished resistor. In subsequent fabrication steps, one or more dielectric and conductive layers may be applied over the layer including over the cured resistors just applied, as shown in FIG. 4. Moreover, additional resistors may be formed directly onto subsequently formed layers of the PCB blank. In a completed PCB, formed by the process, numerous passive resistors may be formed onto each layer of the PCB such that a large number of resistors are embedded within the PCB panel 240.
A significant benefit of the method of embedding passive elements into the PCB is that each embedded passive element eliminates a corresponding surface mounted passive element thereby providing more space on the surface for mounting other elements, or reducing the overall area of the finished circuit board. In addition, the cost of fabricating embedded resistors is lower than the cost of separately fabricating and then surface mounting surface mount resistors. The above process and benefits also apply to other embedded passive elements such as capacitors and inductors.
Another significant benefit of the method of embedding passive elements into the PCB is that the electrical performance of the associated circuits may be improved. Closer placement of the passive circuit elements to other passive and active circuit elements reduces the path length of the conductive interconnects between elements. Especially for circuits operating at high frequencies, short interconnects reduce the radiated electromagnetic fields and reduce parasitic capacitances and inductances present in the circuit.
Embedded elements are usually placed on layers close to the core material because this part of the panel provides the most stable surface during the build-up or lamination process. The initial placement of these elements occurs in the earlier stages of production of a PCB at which point the elements are on the top surface of the panel. After all of the passive elements are printed and cured, or etched, another layer, e.g. a copper or dielectric layer, is laminated over the layer to embed the passive elements.
A significant problem with the use of embedded passive elements is that the electrical characteristics of the deposited or etched elements are difficult to control and predict because of variables in the fabrication process. For many applications, it has not been possible to fabricate embedded elements with sufficient accuracy due to these limitations.
Accordingly, heretofore, embedded passive elements have been restricted to use in circuit paths that can tolerate a wide variation in resistor or other passive element values and manufacturers have been forced to use surface mounted passive elements whenever a narrow range of electrical performance is required of a particular circuit element. This has limited the use of embedded passive printed circuit boards in many critical performance systems.
The majority of the problems associated with embedding resistors within layers of a PCB panel relate to the typical materials used for the resistor elements, the substrate or mounting material, and any surrounding materials proximate to the resistor such as a dielectric layer laminated or coated on top of the resistor.
Two major groups of materials are currently used for embedded resistor elements in PCB panels. The first material group includes thin film resistors made from different metal-based alloys which are laminated or deposited on the board surface. The thickness of these resistors is usually less than 1 micron. The thickness and composition of the resistive layer determine the sheet resistance of the material. The sheet resistance, or resistivity, of the material is given in units of ohms per square. The thinner the material forming the resistive layer, the higher the sheet resistivity. At present, commercially available thin film resistors have a sheet resistivity within a range of 25 Ohms/square to 250 Ohms/square, although recent reports describe a 5 nm thick material with a sheet resistivity of 1000 Ohms/square (See Shipley Inc., Insite™ resistors, Ref. P. Chinoy, et al., CircuiTree Magazine, March 2002, p 78).
Two principal processes are used in the formation of thin film resistors. The first involves lamination of a continuous copper foil/metal alloy sheet, followed by photo-mask, expose, and etch steps to pattern both the copper and thin film material according to the circuit design. This is a fully subtractive process, since material is removed to form the circuit elements. A typical thin film resistor formed by this subtractive process is shown in FIG. 20A. The continuous copper foil/metal alloy sheet is laminated to a dielectric 2010. After the required etching steps, a thin film resistor 2030 is formed on the surface of the dielectric. Copper pads 2020A and 2020B are in contact with the resistor 2030. Typical materials utilized for this first type of thin film resistors include Ohmega-Ply sheets from Ohmega Electronics (See Glen Walther, Tolerance analysis of Ohmega-Ply resistors in multilayer PCB design, CircuiTree Magazine, March 2001, p 64); Ni/Cr or Ni/Cr/Al/Si alloys from Gould Electronics (See Jiangtao Wang and Sid Clouser, Thin film embedded resistors, IPC Printed Circuit EXPO2001. 2001, S08-1-5). Thickness and composition of the resistive layer are typically well controlled in this process.
The second process involves an additive process of selective deposition, typically an electrochemical plating process, to form the resistive elements. As depicted in FIG. 20B, this plating 2040 is performed on the surface of pre-patterned copper on a PCB dielectric substrate or panel 2010 such that it contacts at least two electrically independent copper pads 2020A and 2020B already present at the ends of the resistor according to the circuit design. A typical material utilized for this second type of thin film resistor includes Ni—P electroplated material from MacDermid Inc. (See Joe D'Ambrisi, Dennis Fritz and Dave Sawoska, Plated embedded resistors for high speed circuit applications, IPC Annual Meeting, October 2001, S02-1-4). Thickness and composition of the resistive layer are susceptible to variation due to difficulty in control of the process parameters used to deposit the film.
In both processes, but especially in the second, the variation in thickness and composition of the resistive layer determines in most part the variation in sheet resistance of the material. Furthermore, in both processes, the geometry and dimensions of the film pattern between the copper pads, and the distance between the copper pads, determine the actual resistor value in Ohms. Some, but not all, of these variables can be controlled within reasonable tolerance limits during the PCB fabrication process. Thin film resistors deposited on PCB panels in production show variation in their values on the order of +/−10%.
In the second material group, known as thick film resistors, resistors are formed from pastes deposited onto the PCB panel or a separate material layer. These pastes may be carbon or silver-filled epoxy mixtures or may have different compositions with resistive properties. Two basic processes are used. The first involves lamination following a high temperature cure of screen printed paste on a copper foil, and the second involves a low temperature cure of paste printed on the surface of patterned copper on a PCB dielectric substrate or panel. Thick film processes may involve pastes with sheet resistivity from 15 Ohms/square to 100 kOhm/square, with correspondingly wide variation in material composition.
In the first process, and according to processing instructions for DuPont™ materials (See William J. Borland and Saul Ferguson, Embedded passive elements, CircuiTree Magazine, March 2001, p. 94-106), thick film resistor paste is screen printed onto copper foil at the proper locations prior to firing in an oven in N2 atmosphere at 900° C. After that the pre-printed copper foil is laminated to the dielectric layer of the board, and the resistors are exposed through selective etching of the copper according to the circuit design.
In the second process, the wet resistor paste for embedded thick film resistors is typically screen-printed on the surface of patterned copper on a PCB dielectric substrate or panel 2010 as depicted in FIG. 20C. Alternately, the paste may be selectively deposited in a bead from a dispenser. In both cases, the paste 2050 is so deposited such that it contacts at least two electrically independent copper pads 2020A and 2020B already present at the ends of the resistor according to the circuit design. Additionally, another conductive interface layer may be present between the resistor paste and the copper pads to improve contact between the paste and the copper. The wet paste is then oven cured for durability and stability. Several polymer thick film (PTF) paste compositions with sheet resistivity from 15 Ohms/square to 100 kOhm/square are offered by Asahi Chemical Co. The printing and curing processes are adjusted for each paste type. The paste is printed at the resistor location directly onto pre-etched conductive tracks. The board, including dielectric core, is then fired in an oven for a relatively short period of time. According to Asahi documentation, the curing temperature varies widely from one material to another, with typical values ranging from 150° C. to 270° C. As a result, if several paste compositions are used to produce the range of resistor values required for any particular layer, several cycles of printing and firing are involved in resistor fabrication. During these cycles, one or more pastes will have more than one curing cycle, making the prediction of final resistance values for these resistors quite complex. Furthermore, the low temperature and short time used for the curing process result in a cured paste that is softer and less temperature stable than its high temperature counterpart.
As for thin film resistors, the final resistance of the as-formed thick film resistor is governed by composition of the cured resistive paste, the dimensions and geometry of the paste in length, width and thickness, and the distance between copper contact pads. FIGS. 20D and 20E depict typical cross sections along the width of a thick-film resistor. FIG. 20D shows a resistor with an even width of the resistive material 2060 and two edge zones 2070 that are short compared to the width, whereas FIG. 20E depicts a thick film resistor 2080 with a thickness profile that has a variation from zero microns at the edge to about 50-60 microns in the centre of the resistor. This profile may result from the screen printing process and melting during curing. The non-uniformity of the thickness profile has implications on the predictability of final value and also affects the laser trimming process.
In production, the as-formed thick film resistor process typically provides a resistor with a distribution of +/−20% from the mean value. Additionally, the mean value may be shifted relative to the target as-printed value. The tolerance of as-formed thin film resistor values is better than that of the thick film resistors because better control of material composition and thickness is possible for the thin film materials and processes.
Thick film resistors, although prone to larger variation due to the effects described above, offer other advantages over thin film resistors including at present a much wider range of material resistivity and lower overall material, process, and implementation cost.
It is evident that in order to bring the as-formed resistor tolerances within +/−1% to +/−5% of the target value, trimming of the resistors, as described above, is required. This may be said for all types of both thin film and thick film devices, and as has been discussed above, the characteristics of each may be quite different. Typically, the trimming operation is performed after the resistors have been deposited and are located on the outside of the panel at that process step before lamination or other processes.
Conventional systems used for trimming chip resistors and hybrid circuits are designed to process substrates or panels up to about 8 inches by 8 inches, depending on the application. In addition, these substrates are typically manufactured of alumina ceramic and are temperature stable, rigid and free from distortion.
There is now a need to perform such trimming on larger substrate sizes, such as 18 by 24, or even 24 by 36 inch panels for embedded passive applications. Furthermore, these substrates will typically be printed circuit boards, that are flexible, affected by changes in temperature and prone to distortion in lamination or other thermal cycling steps.
Automation for handling of the typical smaller alumina substrates that are the usual base for discrete passive elements and hybrid circuits is a well developed area. Machines for trimming these types of components often include handlers that feed individual substrates from a stack into the trimming section of the machine before they are moved out to an unloading area. These machines transport the substrates only, and there is no allowance for significant substrate flexibility or distortion either in handling or automated vision alignment of the parts prior to the trimming process.
Within the printed circuit board industry, typical panel sizes range from 12 inches by 18 inches, to 18 inches by 24 inches to 24 inches by 30 inches. These panels can be very flexible, and because the conductive layers can be fragile, so called slip sheets are often placed between panels when they are stacked. These slip sheets are usually thin sheets of plastic or paper. A variety of automated handling equipment is used to transport circuit boards, and to insert and remove slip sheets during conventional printed circuit board manufacturing. These handlers include combinations of conveyors, and lifting/shuttling equipment, typically using vacuum suction cups to pick up individual PCB panels or slip sheets.
A common PCB panel core substrate dielectric material is a glass or ceramic reinforced or filled epoxy resin. The common name used for this material is FR4, although many specific trade names exist for these materials. The initial copper conductor layers, upon which the embedded resistors may be placed, are located on and/or within this core. During PCB fabrication, the core is subjected to many process steps, many of them involving flexing during handling, cycles of large temperature and pressure swings such as lamination, liquid chemical baths such as etching and plating, etc. The epoxy material is prone to softening and creep at elevated temperatures, resulting in distortion of the panel. The epoxy material is also prone to moisture absorption, changing its thermal and electrical characteristics. In turn, resistors located on this core material are thus susceptible to stresses caused by flexing and dimensional changes in the substrate, thermal cycling, and moisture absorption, among other factors. All of these factors contribute to a possible change in the resistance of the element during the process steps required for PCB panel fabrication.
The laser trimming process, whereby material is melted, vaporized, ablated, or photo-chemically altered may affect the material properties of both the resistor and the core or covering materials, if present. The geometry of the resistive element is also typically substantially changed by the trimming process, which can in turn affect the electrical characteristics and behavior of the resistor during thermal cycles, chemical baths, and other processes. It is known both for resistors conventionally formed during chip resistor or hybrid circuit manufacture, and resistors formed on PCB substrates, that the overall stability of a resistor is affected by the trimming process. For the case of resistors formed on the surface of PCB panels, it has been found that short and long term drifts occur during and after laser trimming, and depend on the paste type used as the resistor material. Factors affecting the drift in resistance value after trimming include, but are not limited to: the paste material; the curing cycle parameters; the resistor size, thickness, and geometry; and the form and structure, or morphology, of the resistor surface. Each of these factors can also affect resistance value drifts during other steps of panel production.
Similar to the core materials, the outer layer laminate dielectric materials are typically composed of a polymer resin which is coated or laminated over the core. This layer may then also be proximate to and in contact with embedded resistors, resulting in further modification of the electrical, mechanical, and other properties of the resistors.
Factors that are known to contribute to resistance changes due to lamination include, but are not limited to: duration of heat applied to the panel during lamination or other steps; resistor properties changes due to heating; substrate properties changes due to heating; stresses in resistor and/or substrate that are released due to pressure/heat applied during different steps of lamination; and chemical reactions in the paste material as well as in the substrate material or both, or with interlayer, if present.
In general, resistors made of high-resistivity paste show larger drifts and thermal effects compared to low-resistivity pastes. This is at least partially due to low thermal conductivity of the high-resistivity pastes because of the lower concentration of conductive material (carbon, for example).
Therefore, through all of the process steps involved in the fabrication of a finished PCB panel, there results a change, or drift, in the resistor value. It is known that the drift of resistance values for embedded resistors in PCB panels is larger in magnitude and less predictable than the drift that is conventionally associated with chip resistor or hybrid circuit manufacture, due to many factors including those described above. It is also now a requirement, however, that the tolerance of the resistance value of an embedded resistor with respect to its nominal target value is as tight as that for chip resistors used today, according to the requirements of the circuit.
The average or mean of absolute resistance values and the spread of these values about the mean are typically regarded as statistical quantities that are used to quantify the control possible over the resistor forming process, as well as the effects of the various drift mechanisms discussed above. The spread of values may be characterized by the standard deviation of a gaussian distribution function fit to the data, where the term sigma or σ refers to one standard deviation. It is known from the art of laser trimming chip resistors that the wide 3σ distribution of +/−10% to +/−20% of the as-formed resistor values can be brought down to better than +/−1%.
Since typical laser trimming removes material and thus the resistance of an element can only be increased, the upper limit of the distribution of the pre-trimmed values should lie below the target value to achieve good yield within the target distribution (e.g. +/−1%) after trimming. The effects described previously that contribute to drift also contribute to drift of the resistance values after trimming, since subsequent processes such as lamination may occur. In this case both the final mean and standard distribution of the resistor values may differ from those immediately after trimming. From the descriptions above, it may also be seen that the drift effects on this distribution may be different for different resistor target values, for different resistor sizes and geometries, for different resistor and surrounding materials within a PCB, and for different process parameters used during the PCB panel fabrication.
As has been described above, there may exist several contributors to drift subsequent to trimming and up to final completion of the PCB panel, some not previously known in conventional chip resistor and hybrid circuit manufacture, and these effects may not be consistent for all resistive circuit elements associated with a PCB panel. The ability, then, to achieve narrow final resistor tolerances is thus not assured, compared to conventional chip resistor and hybrid circuit manufacture.
Accordingly, a need exists for an improved technique for trimming of resistors, or other passive circuit elements, embedded within a relatively large multi-layer PCB panel.